Adult female zebrafish, control (top), and exposed to 320 mg/L PTU for 10 days (bottom); H&E
Propylthiouracil interferes with the synthesis of thyroid hormone, thus inducing enhanced secretion of thyrotropic hormone (TSH) by the pituitary. This in turn causes activation of the thyroid follicular cells (lower image), as indicated by the hypochromasy and hypertrophy of the nuclei, and by hypertrophy of the synthetic apparatus (RER, Golgi complex and mitochondria), with a subsequent morphological change of these cells to a columnar appearance (compare with the reference thyroid follicles in the upper image). The follicles are completely or almost depleted of colloid.
The morphology of the rodlet cells remains unchanged.